1. Field of the Invention
The present invention relates to minimizing defects in the components produced by an extreme ultraviolet lithography (EUVL) system, and more specifically, it relates to prevention of wafer and photomask deformation.
2. Description of Related Art
Wafers and photomasks are mounted (clamped) in various tools during their manufacture and their use. A particular type of mount is fabricated by physical vapor deposition (PVD) or chemical vapor deposition (CVD) from ceramic and used electrostatically with what is called the Johnsen-Rahbek effect as disclosed in U.S. Pat. No. 5,117,121. Other fabrication methods include plasma spray deposition, brazing, flame spray deposition and the like. The fabrication and use of these supports are known to generate particulates in various manners on the surfaces of these supports. The supports are also known as chucks.
In use, a wafer rests flush against the surface of the chuck body as a clamping force is generated by vacuum, or a chucking voltage is applied to electrodes on the chuck.
If the mounting surfaces of the tools allow any particulate to become entrapped between the mounting surface and the wafer or mask, then the wafer or mask may be deformed by the entrapped particle. For example, if the backside of a wafer is to be clamped, by vacuum or electrostatically against a flat reference surface, the entrapped particle could cause a deformation of the front side of the wafer, which will therefore not lie in a flat plane. There are many approaches to reducing the probability of trapping particulates including the use of a xe2x80x9cpinxe2x80x9d chuck that has a real area of contact that is about 1% of the nominal area of contact.
U.S. Pat. No. 5,656,093 is directed to a metal interrupted coating on the chuck surface that is less abrasive and compliant than the chuck surface itself. The purpose of the coating is to reduce the generation of debris on the backside of a wafer from tens of thousands of particles to hundreds of particles. The invention creates a distance between the underside surface of the wafer and the chuck as defined by the thickness of the support members, and requires that the distance be larger than the expected diameter of the contaminate particles so that the particles do not adhere to the underside of the wafer during processing. The patent indicates that the interrupted pattern allows a volume where debris can collect. The number and spacing of the pads must be judiciously (evenly) positioned to optimize support yet limit their interference with the clamping force. Cooling is provided by an ancillary heat transfer medium.
U.S. Pat. No. 6,019,166 teaches a pickup chuck having a heat slug for removing heat from a heat-generating device that is held within the pickup chuck. The patent indicates that the compliant layer cushions the backside, thereby reducing the likelihood of damaging the integrated circuit.
U.S. Pat. No. 6,117,246 discloses a nonabrasive conductive polymer pad for supporting a workpiece upon a workpiece support surface of an electrostatic chuck. The disclosure defines the conductive polymer pad as a polyimide or a fluoropolymer, and the like, with a controlled resistivity of about 107 to 1012 ohm-cm and arranged in a plurality of islands and connector strips, optionally interconnected. The invention seeks to avoid particle generation due to abrasive contact. As with U.S. Pat. No. 5,656,093, the thickness of the stand-off pad should preferably be larger than the expected diameter of contaminant particles to avoid contaminant particles from adhering to the backside of the wafer during processing.
Studies have shown that a 10-micron particle on a flat chuck can displace a reticle (i.e., a test wafer) surface for a radial distance by an inch or more. The actual height and diameter of the particle-induced displacement is dependent on numerous parameters such as the particle size, the particle hardness, the clamping force and the reticle thickness. The radial distance that a particle will displace the reticle surface can be reduced by either making the reticle very thin or making the reticle very thick. In both cases, a high clamping force is desirable. If the reticle is thin, the reticle will bend to conform to the particle and the surface displacement will drop off quickly. If the reticle is thick, the reticle will not bend but rather plastically deform at the point of contact and the particle will be embedded in the reticle.
The likelihood of particle reduction on the substrate is influenced by the size of the particle being removed (smaller particles are less likely to be prevented while larger ones can be washed or filtered out). It would be more desirable to reduce the effect of the particle on surface deformation once it becomes entrapped, thereby allowing smaller spacing between the chuck and workpiece.
It is an object of the present invention to provide a complaint layer for supporting a wafer, or other workpiece, in a spaced-apart relation to a chuck that adsorbs contaminant particles that could create deformation in the workpiece.
It is another object to provide a method for using a chuck having a compliant layer.
These and other objects will be apparent to those skilled in the art based on the disclosure herein.
Compliant materials (e.g., polymers) allow particles to embed themselves into the material but also act rigidly to allow a wafer to conform to the lapped surface. The spacing between the conformal layer and the workpiece can then be on the same order as the expected particulates and thereby avoid the necessity for higher spacing of other chucks.
For manufacturing ease, the compliant layer may be built as an interrupted surface, e.g., a plurality of islands or a fullxe2x80x94contact surface.
The compliant polymer material is different from the chuck material and must be conformal to ensure the compliant layer does not alter the overall form of the surface.